Much of the tall-grass prairie ecosystem has been converted to agriculture. In order to conserve biodiversity and enhance educational opportunities St. Olaf College has restored 61 ha of agricultural land to prairie since 1989. Fires, the main historical disturbance in prairies, reduce non-prairie species and provide openings to maintain the full diversity of prairie species. Each section of St. Olaf prairie is burned every 2-5 years and since prairie sections were planted with a similar species mix and burned in different years comparisons can be made among the sections. Within each of ten different study sections, burned in the current year or previous years, seven random points were chosen for soil sampling. Two soil cores were taken at each point to measure the following chemical and physical characteristics: nitrates, ammonia phosphates, percent organic matter, percent moisture, and bulk density. Separate soil cores were taken for nucleic acid extractions to assess presence and expression of microbial genes involved in nitrogen pathways. From vegetation plot samples at three of the seven points in each section species were identified and separated into grass, legume and non-legume functional groups to determine dry weight biomass.
This study shows that more recently burned prairie sections had higher species diversity and higher levels of soil moisture, organic matter, and nitrates. Changes in levels of phosphates and ammonia as well as soil bulk density were not significantly different between the burned and unburned treatments. Over the course of the growing season both nitrate and ammonia levels increased across the entire study area while phosphate levels showed no statistical difference over time. Additionally, the species composition of prairie sections burned in the current year was different from those burned in previous years. While the burned areas contained more grasses and slightly more legumes, the unburned patches were dominated by forbs. These results show that burn cycles can be managed to encourage maximum diversity of either grasses or forbs under certain conditions. The burns allowed more light and moisture to reach the seed bank as the fire removed the large duff layer. This, in addition to the increase in organic matter created by burning dead plant material and variation in soil types, can lead to a diversification of the prairie community both biologically and chemically.